A known measuring device can be seen in US 2006/0208171 A1. This measuring device serves to determine a vegetation index value of plants; in particular this known measuring device is intended to determine the so-called REIP (“Red Edge Inflection Point”) vegetation index. Plant measurements of this type have the purpose of enabling the utilization of the obtained measurement quantities for a determination of the most important characteristic values of the plant, namely, in the case of the REIP vegetation index mainly for determining the current nitrogen content of the plants being measured; based on the determined nitrogen content it is then possible to draw up a suitable fertilization plan for the field in question; in practice, e.g., corresponding GPS-supported fertilization systems are already being used which employ the determined nitrogen values to optimally supply fertilizer to specific surface areas.
The known vegetation index measurements are based on the light absorption or reflection characteristics of plants as shown in FIG. 3: According to this, the plants possess the general property of absorbing light of certain wavelengths (namely, <700 nm) while reflecting light of longer wavelengths (i.e. >800 nm). As may be seen from FIG. 3, the blue, green and red light components are absorbed by the leaves of the plant, with the cell structure and the water content of the plant causing a shift from absorption to reflection along a steep flank (“red edge”) in the beginning infrared range.
Investigations have shown that this transitional area of the red edge (“Red Edge Inflection Point”—REIP) may be utilized for determining the chlorophyll content and the nitrogen content of plants. Namely, there exists the interrelation between the plant's REIP and its nitrogen content as shown in FIG. 4, and it could be demonstrated by Guyot and Baret (1988) that four measurements each with different wavelengths are sufficient for determining the nitrogen content.
In document US 2006/0208171 A1 mentioned at the outset it is therefore proposed to provide four light-emitting elements having the form of light-emitting diodes (LEDs) for the measurement of the REIP value and thus of the nitrogen content, each of which emits substantially monochromatic light of a predetermined wavelength within the REIP range (i.e., in the range between 660 and 780 nm); a control means successively controls these four light-emitting diodes in a cyclical sequence, with the respective intensity of the reflected light being determined based on the output signal of a light sensor element, and the currently valid vegetation index value or REIP value lastly being calculated based on the determined intensities of the overall measuring cycle.
In this known measurement device the light sensor element used is a photodiode whose analog and moreover highly noisy output signal must be subjected to complex processing (phase detection) and must then furthermore be subjected to an A/D conversion for the further calculation. Studies have shown that this strongly affects the attainable measurement accuracy; in addition the hardware expenditure is comparatively high on account of the additionally required phase detector and A/D converter.